Consider a classical system consisting of \(N\)  particles with the density \(\rho \)  being bounded by a potential U(\(\vec{r}\) ) [1]}. The system is said to be stable if the characteristic time of its evolution is long relative to the characteristic time of the system. The system may then be assumed to be in equilibrium with a distribution function \(f\)  depending on the phase space coordinates. In case of symmetries, one usually choose coordinates adapted to these symmetries taking care of the distribution function which is a scalar density rather than scalar. With (\(\vec{x},\vec{p}\) ) being the coordinates in the phase space we have
\(N=\int f(\vec{x},\vec{p})~ d^3p ~d^3x.\) 
